In patients with HIV and other immunosuppressive diseases, one strategy for preventing infectious processes has been chemoprophylaxis or long-term suppression. However, drug toxicity and development of pathogen resistance have made this approach less attractive, as has the complexity of resulting drug regimens. Another approach would be detected of signal in serum or buffy coats by polymerase chain reaction (PCR) for pathogens such as mycobacterium-MAC avium intracellulare, cytomegalovirus (CMV), TB, PCP, and perhaps others. PCR is being adapted for rapid screening. Initial efforts will involve CMV and MAC. Laboratory samples are being tested, and quantitative standards developed. A second stage of development will involve human samples. A third stage will involve prospective monitoring of patients. This strategy is a major change from the current emphasis on expensive and toxic drug regimens that are difficult to comply with. Proper probes, sample types, and populations need to be defined.